1. Field of the Invention
This invention relates to FM stereophonic receivers which comprise a muting circuit operating to interrupt or reduce its reproduction output according to an electric field strength, and a mode changing circuit operating to change its reproduction mode to a stereophonic reproduction mode or a monaural reproduction mode selectively according to the electric field strength, and more particularly to an FM stereophonic receiver in which the muting operation point of the muting circuit and the stereophonic and monaural change-over operation point of the mode changing circuit are switched, for instance, by one manual change-over switch.
2. Description of the Prior Art
As is well known in the art, the electric field strength of an FM broadcasting wave is greatly changed by the effects of buildings and geographical features lying in the course of the wave because the FM broadcasting wave is propagated more straightly than the AM broadcasting wave. Therefore, in the case when the FM broadcasting wave is received by a tuner installed in a vehicle, the reproduction of the FM broadcasting wave suffers from disadvantages such as occurrence of a so-called skip noise or is liable to be affected by multi-path. The FM broadcasting is excellent in high fidelity reproduction; however, as was described above, the FM broadcasting wave reproduction is liable to be affected by a variety of interferences or noises, and therefore it is necessary to provide countermeasures against the effects of such noises. One of the countermeasures considered is a method in which in the case where the electric field strength is sufficiently high (for instance, higher than 40 dB), the reproduction is effected in a stereophonic mode, and when the electric field strength is somewhat lowered, the stereophonic mode is automatically changed to a monaural mode, thereby to maintain the S/N ratio (the ratio of signal to noise) thereof at an allowable value. Furthermore, it is preferable that when the electric field strength is further lowered (for instance, lower than 20 dB) the signal reproduction is automatically suppressed or reduced.
The matters described above are applicable to the case where music is enjoyed through the reproduction of FM broadcasting waves. However, there may be persons who like to enjoy music in the stereophonic mode even if the S/N ratio is somewhat worse. In addition, in the case where the FM broadcasting service is utilized as an information source as in the case of listening to local traffic information or weather forecasts by the FM broadcasting, it is necessary to provide a means which, even if the S/N ratio is considerably lowered, operates not to intertupt the reproduction of audio signals.